The invention relates to improvements in calenders in general and to improvements in methods of operating calenders. More particularly, the invention relates to improvements in calenders (especially supercalenders) and to improvements in methods of operating calenders of the type wherein a top roll and a bottom roll flank one or more intermediate rolls and the bottom and/or top roll is deformable so that it can select the profile of the nip between such deformable roll and the neighboring roll.
It is well known to provide a calender with a deformable top roll and/or with a deformable bottom roll and to operate the calender in such a way that the bottom roll is first caused to close the lowermost nip, thereupon the next-to-the-lowermost nip, and so forth. Such closing of the nips starting at the bottom and proceeding toward the top takes place after the material (such as a web of paper) has been properly threaded through the nips and is trained over one or more guide rollers or the like. As a rule, calenders of the just described character are used for the treatment of paper webs; however, they can also be used for the treatment of other materials such as plastic foils or the like. As used herein, the term calender is intended to embrace all kinds of machines (including smoothing, printing and rolling machines) wherein top and bottom rolls flank one or more intermediate rolls and define therewith nips for the passage of paper webs or other materials which are to be treated in the nips as a result of the application of heat and/or pressure.
Commonly owned U.S. Pat. No. 4,625,637 to Pav et al. discloses a roll assembly for use in calenders wherein the top and bottom rolls have deformable cylindrical shells and flank a set of intermediate rolls. The shells of the bottom and top rolls are rotatable about and include portions which are movable radially of non-rotatable carriers. The shells confine rows of primary deforming elements in the form of hydrostatic bearings which can be operated to change the shape of the shells as well as to urge selected portions of the shells against the adjacent intermediate rolls with a given force. Each shell further confines at least one row of secondary deforming elements which are disposed diametrically opposite the respective primary deforming elements. Each deforming element includes a cylinder and piston unit and has a hydrostatic bearing member which is adjacent the internal surface of the respective shell. Adjusting means is provided to regulate the pressure in selected cylinder and piston units and to thus select the force with which the corresponding portions of the shell bear upon running webs in the respective portions of the nip. This patent further discloses the possibility of heating one or more hard intermediate rolls.
The axes of intermediate rolls in the patented calender are bent, mainly as a result of the action of overhanging weights upon their ends. Therefore, the central portions of the rolls bulge upwardly and are convex from above and concave from below when the calender is not in use. This renders it necessary to conform the shapes of the top and bottom rolls to the shapes of the adjacent intermediate rolls in order to achieve uniform load distribution along the nips. When the threading of a web through the nips is completed and the width of the nips is reduced, starting with the lowermost nip and proceeding toward the nip of the top roll and the adjacent topmost intermediate roll, the upwardly bent median portion of the topmost intermediate roll is caused to engage the web at the center of the top roll while such center of the top roll has a straight horizontal axis or sags downwardly under the weight of the median portion of the top roll. Attempts to avoid a mere point contact between the shell of the top roll and the web in the nip of the top roll with the adjacent topmost intermediate roll include the application of pressure to those primary deforming elements in the shell of the top roll which are adjacent the axial ends of such shell. This necessitates the application of high pressures, i.e., the pressure in the nip of the top roll with the adjacent intermediate roll is very high which is often undesirable for a number of reasons. In addition, the pressure in the top nip is raised as a result of application of external forces. The nip load increases from nip to nip in a direction from the uppermost nip toward the nip of the bottom roll and the adjacent lowermost intermediate roll. Such linear increase of nip load is due to the fact that each lower roll must bear the weight of the roll or rolls above it. High nip loads are undesirable in connection with the treatment of many types of materials. For example, if the nip load is high, satisfactory gloss, smoothness and receptivity to print can be achieved only as a result of pronounced densification of the material.
Another undesirable effect of pronounced nip loads is that they shorten the useful life of certain types of rolls. For example, if the top roll is adjacent an intermediate roll which has an outermost layer of elastic material, the initial mere point contact with the top roll and the immediately following pronounced increase of nip load in the nip of the top roll with the elastic intermediate roll can entail immediate pronounced or even irreparable damage to the intermediate roll. Many calenders employ an elastic intermediate roll immediately adjacent the top roll. The situation is not improved if the topmost intermediate roll is initially engaged by a relatively short central portion of the shell of the top roll, i.e., if a mere point contact is replaced with a short linear contact.
Calenders with deformable top and/or bottom rolls are also described in U.S. Pat. No. 4,480,537 to Agronin et al., in the April 1986 Tappi Journal (pages 88-94) and on pages 976-982 of WOCHENBLATT FUR PAPIERFABRIKATION (23/24, 1986).